Texting with my kid brother?

Hi everyone,

So I have a 7 year old little brother who lives in California (I live in Oregon, 27) and I want to be able to keep in better touch with him without having to go through our mother all the time. He's super tech-savvy for his age, but I don't know of a safe device to get him to be able to keep in touch. I want some kind of device where we can text each other whenever (not bound to distance, since I live a state away), that also prevents strangers from contacting him or prevents him from accidentally contacting strangers. He has an old iPod Touch, but that's it. Does anyone have any suggestions of devices or apps that could be installed on his iPod Touch that would fit the situation?